A thermocouple is really a popular sort of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications for their relatively low cost and wide measurement ranges. Especially, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made from two different metal alloys. The conductors are normally that are part of a cable using a heat-resistant sheath, often with the integral shield conductor. At one end of your cable, both conductors are electrically shorted together by crimping, welding, etc. This end from the thermocouple–the new junction–is thermally linked to the object to become measured. Another end–the cold junction, sometimes called reference junction–is linked to a measurement system. The objective, of course, is to discover the temperature near the hot junction.
It must be noted that the “hot” junction, that is somewhat of your misnomer, may in reality attend a temperature lower compared to the reference junction if low temperatures are measured.
Since thermocouple voltage is actually a purpose of the temperature distinction between junctions, it really is essential to know both voltage and reference junction temperature in order to determine the temperature with the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to preserve it in a fixed, known temperature.
You will discover a misconception of methods thermocouples operate. The misconception is that the hot junction is definitely the source of the output voltage. This can be wrong. The voltage is generated across the size of the wire. Hence, when the entire wire length is at a similar temperature no voltage could be generated. If this type of were not true we connect a resistive load to a uniformly heated thermocoupler inside an oven and make use of additional heat in the resistor to create a perpetual motion machine from the first kind.
The erroneous model also claims that junction voltages are generated on the cold end between the special thermocouple wire and the copper circuit, hence, a cold junction temperature measurement is necessary. This idea is wrong. The cold -end temperature is the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems opt to measure, instead of control, the reference junction temperature. This is due to the fact that it is usually cheaper to merely put in a reference junction sensor with an existing measurement system rather than to add-on a full-blown temperature controller.
Sensoray Smart A/D’s look at the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a special channel to this particular function serves two purposes: no application channels are consumed with the reference junction sensor, along with the dedicated channel is automatically pre-configured with this function without requiring host processor support. This special channel is made for direct link to the reference junction sensor that may be standard on many Sensoray termination boards.
Linearization In the “useable” temperature range of any thermocouple, you will discover a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. Actually, most thermocouples are incredibly non-linear over their operating ranges. As a way to obtain temperature data from the thermocouple, it is actually necessary to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is referred to as “linearization.”
Several methods are typically employed to linearize thermocouples. On the low-cost end from the solution spectrum, one could restrict thermocouple operating range such that the thermocouple is almost linear to in the measurement resolution. At the opposite end of your spectrum, special thermocouple interface components (integrated circuits or modules) are for sale to perform both linearization and reference junction compensation within the analog domain. In general, neither of these methods is well-suited for inexpensive, multipoint data acquisition systems.
As well as linearizing thermocouples within the analog domain, it is actually possible to perform such linearizations from the digital domain. This can be accomplished by means of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or occasionally a hybrid of these two methods.
The Linearization Process Sensoray’s Smart A/D’s hire a thermocouple measurement and linearization procedure that was created to hold costs to a practical level without having to sacrifice performance.
First, both thermocouple and reference junction sensor signals are digitized to have thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized at a higher rate in comparison to the reference junction because it is assumed that the reference junction is relatively stable compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.